The present invention relates in general to amplifier circuits and, more particularly, to a transimpedance audio amplifier for providing variable power levels while maintaining a constant complex load impedance.
Audio or instrument amplifiers have long been used to control the sound amplification, equalization, filtering, special effects, and other signal processing of the audio signal. The audio signal is generated by an audio source and then amplified and filtered by the power audio amplifier. Common examples of the audio source include a musical instrument, such as a guitar, a receiver and tuner, and a microphone. The power audio amplifier is typically a separate unit from the audio source. The amplified and filtered audio signal then drives a loudspeaker.
Audio amplifiers are available in a wide assortment of design technologies, power amplification capabilities, frequency responses, enclosures, and price ranges; each offered with particular end applications in mind. There are electronic or solid state power audio amplifiers, and there are vacuum tube power audio amplifiers, with output power ratings from less than 10 to more than 100 watts, over a frequency range of approximately 20 to 20 K hertz. In the music industry, and more particularly, for musical instruments such as guitars, the vacuum tube power audio amplifier offers certain desirable characteristics for the end user or artist. As popular music has evolved, some musicians, including guitarists, have developed playing styles that involve intentionally overdriving their power audio amplifiers into distortion or nonlinear region to achieve a unique sound effect. The vacuum tube power audio amplifier offers certain inherent physical and design inaccuracies that allow or aid in the generation of the distortion within its frequency response.
The desired distortion effect is generally attainable only when the power audio amplifier is operated at substantially high volume levels. Unfortunately, other people within earshot do not always have the same appreciation as the artist for his/her music. Thus, at times, the artist is prevented from playing the overdrive style because of the loudness, disturbance, or annoyance perceived by others. Although the artist can use earphones if they desire that others not hear the music, such devices can be uncomfortable or inconvenient to wear and tend to restrict movement. Earphones do not have the same sound quality and acoustic properties as a high performance loudspeaker. Moreover, there can be damage to the inner ear from long term exposure to exceptionally loud music or sounds.
In the prior art, some instrument amplifiers provide preamplifier distortion with a master volume control as a way of playing in an overdrive style while maintaining a reasonable acoustic volume level. However, preamplifier distortion generally sounds quite different from power audio amplifier distortion, particularly the variety of amplifier distortion that is characteristic of the vacuum tube power audio amplifier. The artist does not achieve the same sound effects as provided through power audio amplifier distortion.
Another known approach is to place a passive attenuator, such as a T-pad, between the power audio amplifier and the loudspeaker. The vacuum tube power audio amplifier has a complex output impedance, i.e. with resistive and reactive components. Likewise, the loudspeaker has a complex impedance, which is matched to or interacts with the complex output impedance of the power audio amplifier in a manner greatly affecting the overdrive distortion characteristics of the power amplifier-loudspeaker system. The passive attenuator isolates the complex impedance of the loudspeaker from the complex output impedance of the power audio amplifier. When using a passive attenuator, the power audio amplifier is no longer driving the loudspeaker, but rather it is driving the passive attenuator. The passive attenuator between the power audio amplifier and the loudspeaker creates a mismatch or otherwise compromises the interaction between the amplifier output impedance and the load impedance and interferes with the desired distortion sound effects.
A need exists for an audio or instrument amplifier that produces a level of distortion at many different acoustic volume levels while preserving the interaction of the power amplifier output impedance and the loudspeaker impedance.